The objective of this research is to define the present status of immunity to infection with Hl influenzae, type b (HIB). Consideration will be given to elucidating the role of antibodies in the pathogenesis of such infections and to examining the immunological response to infection with this organism. The plan of investigation proposes to study the following problems: 1. To determine the prevalence of antibodies (by bactericidal, hemagglutinating and radio-immuno assay) to HIB among healthy individuals of several age groups. 2. To explore further the variable susceptibility of strains of HIB to serum bactericidal activity. 3. To study the serologic responses to natural infection with HIB and to immunization with HIB polysaccharide capsular material (PRP). 4. To study the colonization with HIB and the formation of antibodies in families of children with HIB infection.